Operation of silicon carbide resistors in protective atmospheres



1931- R. c. BENNER ET AL 1,837,179

OPERATION OF SILICON CARBIDE RESISTORS IN PROTECTIVE ATMOSPHERES Filed July 30, 19:50 2.Sheets-Sheet l II II R m a W/ NWKK ETw o EMA a S EMW mf .N

TMEWCW m mm T Y R T V O M NREL I so 2 Sheets-Sheet 2 OPERATION OF SILICON CARBIDE RESISTORS IN Dec. 15, 1931.

INVENTORS RAYMOND QBENNER GEORGE J. EAsTER BY ARENQE E.HAWKE ATTORNEY Patented Dec. 15 1931 UNITED STATES PATENT OFFICE RAYMOND C. BENNER GEORGE J. EASTER, F NIAGARA FALLS, NEW YORK, AND

CLARENCE E. HAWKE, OF METUCHEN, NEW JERSEY, ASSIGNORS TO THE CARBOBUN- EW YORK, A CORPORATION OF PENNSYL- 7 DUI COMPANY, OF NIAGARA FALLS, N

VANIA OPERATION OF SILICON CARBIDE RESISTORS IN PROTECTIVE ATMOSPHEBES Application filed July 30, 1930, Serial No. 471,714, and in Canada March. 20, 1829.

This invention relates to the prolongation of the useful life of silicon carbide resistors, and to a method of operating such resistors whereby high temperatures can be maintained 3 over prolonged periods of time Without causing undesirable changes in electrical properties. The present application is a continuation in part of our co-opending application, United States Serial No. 324,416, filed De- 10 cember 7, 1928.

The use of silicon carbide heating elements of a rigid or self-sustaining character in electric furnaces is well known. These resistors offer a number of advantages over the other types of elements in common use. They are suitable for operation at temperatures considerably in excess of that possible with a metal resistor such as nichrome. The resistor material has a much higher specific resistance than graphite, so that elements having a fairly large surface for the dissipation bf heat can be operated on the stand ard line voltages, without the use of low voltage transformers.

Silicon carbide resistors are of such a character that they have always been regarded as stable in air. In contrast with graphite resistors, which are subject to burning and rapid mechanical disinte ration when operated in 33 air at temperatures above a dull red heat, a

silicon carbide resistor can be operatedcontinuously in the open air at temperatures up to 1300 C. for more than 1000 hours without any visible change, except for possibly a slight change in color. The silicon carbide resistor is practically the only element which can be operated in air above 1100 0., since graphite, tungsten, molybdenum and other commercial materials of high melting point all oxidize so rapidly that their operation at high temperatures without protection is impossible. In the case of graphite, the material actually burns and is reduced to ash unless protected from oxidation, so that the necessity of protection is evident. However, silicon carbide resistors have been regarded as being of a different type from the easily oxidizable heating elements, and because of their comparative stability they have always been operated without protection since their first introduction into commercial use.

Although silicon carbide resistors possess many advantages, there are certain undesired changes in electrical properties which take place during the life of the element as ordinarily operated. The electrical resistance does not remain constant during use, but is continually changing throughout the life'of the resistor. This change takes place even when the element is operated at fairly low temperatures, as for example 700800 C., although the element is physically and mechanically stable at much higher temperatures. The increase in resistance during the first few hours of operation is often appreciable, and after 1000 hours the increase may be as much as 50 to 100 per cent. In order to counteract the changing resistance it is necessary to continually alter the voltage across the terminals of the resistor if a constant power input is to be maintained.

The temperature coetficient of resistance also changes during the life of the element, the coefficient becoming considerably more negative as the element increases in electrical resistance. This factor is of considerable importance in the operation of an electric furnace, since a strongly negative temperature coeflicient makes the regulation of the power input and the maintenance of a constant temperature very difiicult. In operating the element at a definite voltage, any increase in temperature will cause an increase in current, and the increased current will cause a still further increase in temperature, these increases being cumulative intheir effect. In a very few minutes both the resistor and the contents of the furnace can be damaged or completely destroyed by what was originally only a slight temperature fluctuation.

Although the difiiculty of variable rather than constant electrical properties has always been considered as inherent in the operation of this type of resistor, we have found that the electrical properties of a silicon carbide element can be kept practically constant over prolonged periods of time by operating the resistor in a non-oxidizing atmosphere. In

our improved method of operation, both the original resistance of the element and the original temperature coefficient of resistance are preserved by operation under non-ox1d1z ing conditions. In addition, the useful life of the resistor is greatly prolonged over that which has heretofore been regarded as satisfactory for a resistor of this type. The factor limiting the useful life of a silicon carblde heating element has always been that of increased electrical resistance and change of resistance-temperature characteristics ratherthan physical or mechanical disintegration. In the past it has been customary to operate the resistor without protection until the electrical resistance increased from 50 to 190 per cent, whereupon the element was considered unfit for further use and was discarded.

Recent developments in connection with the manufacture of silicon carbide resistors have made possible the production of an element in which the temperature coeflicient of resistance is positive within the operating range. In several co-pcnding applications (U. S. Serial No. 324,418, filed December 7, 1928; U. S. Serial No. 324,419, filed December 7 1928; and U, S. Serial No. 485,998, filed October 2, 1930) we have disclosed methods of utilizing this property to maintain a practically constant temperature within an electric-furnace. The electrical resistance of this type of element increases as the temperature is increased from approximately 800 C. to 1500 C., and as the furnace becomes overheated, the power input at a given voltage is considerably decreased. On the other hand, if the interior of the furnace is cooled by the introduction of a cold charge, the power input is increased until the furnace returns to normal temperature. A degree of regulation is afforded which often amounts to from 30 to 40 per cent of the power input.

the regulation of the power input by this.

means is reduced to the point Where it is of little practical value. After the element has increased considerably in resistance, the effeet due to the positive temperature coefficient is often completely lost, and the temperature coefiicient may even become negative throughout the entire range of operating' temperatures, thus causing a reverse effect from that desired.

Our method of operation in which a pro tective atmosphere is used affords a means of preserving the original positive temperature coeflicient of resistance of the element, and thus makes available the regulation of both power Input and temperature throughout the useful life of the resistor.

We believe that the maintenance of constant electrical properties by operation in a non-oxidizing atmosphere may be explained as being due to the prevention of the'small amount of oxidation which normally takes place during the operation of the element in air. Although the actual amount of oxidation is small from a physical or mechanical marked effect onelectrical properties. R81

sistors have. been observed in which the electrlcal resistance had increased appreciably,

but the actual amount of oxidation was so small that the surface of the silicon carbide crystals possessed an irridescent appearance characteristic of thin interference films. As

is well known, anoxide film capable of producing interference phenomena has a thickness of the order of magnitude of thewave lengths of visible light.

By the use of a protecting atmosphere, a silicon carbide resistor can also be operated at a considerably higher temperature than has" heretofore been possible with this type of element without afli'ecting the electrical properties. The upper temperature limit for a silicon carbide resistor has previously been regarded as being approximately 1400- 1500 C., and even at this temperature the increase in resistance and change in temperature coeflicient of resistance is very rapid when the element is operated in air.

In the operation of resistors composed of silicon carbide, they have'occasionally been found to warp or bend, even when noappreciable softening of the resistor material takes place. In the operation of resistors in a nonoxidizing atmosphere, this phenomenon has not been observed. It seems probable that the'warping is due to differential expansion caused by localized oxidation. It is known that silicon carbide refractories often expand when exposed to oxidizing conditions.

For the protection of the resistors, any gas .which'will not react with silicon carbide at thetemperature of operation of the ele ent may be used. We prefer to use carbon onoxide oran atmosphere containing an appreciable proportion thereof, although nitrogen or other gases having less reactivity th n ordinary air may be used. If desired, t e resistor may be embedded in a solid protecting material, as for example carbon in the form of lamp black or powdered coke. If an inert gas rather than a solid embedding have a considerable amount of ash, so.'that'it' is desirable to use purer forms of carbon such as lamp black or petroleum coke for the'removal of the oxygen from the interior or the muflle. thin layer of lamp black on the floor of the-muffle will remain for several days without being consumed, if reasonable precautions are takennot to expose the heat ing chamber .to the open air.

A preferred method ofcarrying out our invention is illustrated in the accompanying drawings, in which:

Figure 1 is a vertical section of a form of furnace in which the muflle is subjected to a non-oxidizing atmosphere, the resistors being directly in the muflle;

Figure 2 is a vertical section of a modified furnace in which reducing gas is introduced directly into the heating chamber; and

Figure 3 is a vertical section through the center of the furnace, taken on the line 3-3 of Figure 2.

Referring to the drawings in more detail, the resistors 2, which are composed principally of silicon carbide, are mounted in the usual manner, using either self-cooled'or watercooled terminals. In order to secure uniform heat distribution around and beneath the charge, the upper portion of which is exposed to direct radiation, it is desirable that the lining 4 be composed of highly conductive refractory, as for example, one having a thermal conductivity in excess of .006 calorie/crn"/sec/ 0., as disclosed in our copending application, U. S. Serial No. 324,419, filed December 7, 1928. Silicon carbide or fused alumina are examples of such refractories. Y

The success obtained in maintaining a nonoxidizing atmosphere within the mufile will depend upon the degree to whichthe infiltration of air is prevented. For this reason, it is desirable to use a dense or comparatively impermeable refractory, or else a porous refractory surrounded by granular or powdered carbon or other-carbonaceous material.

In the form of furnace shown in Figure 1 the resistors 2 are placed adjacent to a porous wall 3 of highly conducting refractory such as recrystallized silicon carbide, which conducts heat to granular carbon 5 within the wall of the furnace. The carbon is also shown as extending down through the side walls of the furnace, where it is separated from the interior by the wall 4 of similar material to that composing the wall 3. The silicon carbide is an excellent conductor of heat, so that the outer portion of the .wall may be operated at incandescence,

and any air filtering into the muflle, through the porous walls wlll first come in contact with the hot carbon and the oxygen will be removed therefromto form carbon monoxide. The granular or powdered carbon will afford effective thermal insulation between the in-' ner and outer walls" of the furnace. Fresh carbon may be added' 'through an" opening which is ordinarily closed by'the' plug fi, or

through the o'pen'ings= 7. If desired, a's'xnall quantity *of finely 'divided' carbon may be placedwithin 'the heatingi chamberw Figures'2 and 3 "show the direct use of a non-'rea'ctin gas independent of granular carbon to a 0rd protection for the resistors. Producer gas or other suitable gas is introduced into the muflle through conduits 11, which are preferably locatednear the ports where the resistor terminals penetrate the furnace wall. The flow of gas may be C0117 trolled by means of valves 12.

In the furnaces described above, in which the resistors are placed within the heating chamber, the non-oxidizing atmosphere serves to protect both the resistors and the furnace charge. This factor is particularly important in the case of heat-treating work. -where the metal must be protected from scaling. There are many other instances where it is important to protect the ware itself as well as the resistors.

The means which we have described for carrying out our invention may of course be varied according to the design and type of furnace in which the resistors are employed. Our invention may be defined as being within the scope of the following claims.

1. In an electric furnace, a chamber for ware to be treated, silicon carbide resistors within said chamber, the walls of said chamber being composed of refractory which is permeable to gases, and a granular carbon packing around said chamber walls.

2. In an electric furnace, a chamber for ware to be treated, silicon carbide resistors within said chamber, the walls of said chamher being composed of refractory which is permeable to gases and possesses a high thermal conductivity, and a granular carbon packing around saidchamber walls.

3. The method of operating a silicon carbide resistor which comprises surrounding the resistor with a non-oxidizing atmosphere.

4. The method of operating a silicon carbide resistor which comprises surrounding the resistor with an atmosphere having a substantial proportion of carbon monoxide.

5. An electric furnace comprising in comhination a resistor consisting mainly of silicon carb ide, an enclosing mufiie wall about the resistor, and'means for supplying a nonoxidizing atmosphere around said resistor 5 and. about Ware under treatment in the muffle.

6. In an electrical resistance furnace a silicon carbide resistor, an adjacent muffle whosewalls are composed of silicon carbide, andmeans for maintaining a non oxidizing {t atmosphere around the resistor and muflle.

In testimony whereof we aflix our signat n I RAYMOND o. BENNER.

I GEORGE J. EASTER. I I 15 CLARENCE E. HAWKE: 

